1. Field of the Invention
The present invention relates to a method for producing a Group III nitride-based compound semiconductor device. More particularly, the present invention relates to a method for producing a Group III nitride-based compound semiconductor device employing the so-called laser lift-off technique, in which a Group III nitride-based compound semiconductor is epitaxially grown on a substrate made of a material different from Group III nitride-based compound semiconductor (hereinafter the substrate may be referred to as a “hetero-substrate”) to form a device structure; a conductive support substrate is bonded, via a conductive layer (e.g., a metal layer or a solder layer), to the uppermost layer of the device structure; and the hetero-substrate is removed by decomposing, through laser irradiation, a Group III nitride-based compound semiconductor thin layer in the vicinity of the interface between the Group III nitride-based compound semiconductor and the hetero-substrate. The present invention is particularly effective for a method for producing a Group III nitride-based compound semiconductor light-emitting device in which an electrode of low contact resistance is formed on an N (nitrogen)-polar surface of an n-type layer that, together with a p-type layer, sandwiches a pn junction structure or an active layer.
2. Background Art
With the laser lift-off technique developed by Kelly, et al. (described in Appl. Phys. Lett., vol. 69, 1996, pp. 1749-1751), in a Group III nitride-based compound semiconductor device (e.g., a light-emitting device), the substrate used for epitaxial growth of a Group III nitride-based compound semiconductor can be replaced with a conductive support substrate. This technique realizes, for example, formation of an electrode on the bottom surface of a support substrate of a light-emitting diode. Also, this technique realizes production of a light-emitting device having facing opposite electrodes (i.e., positive and negative electrodes) on both the bottom surface of a substrate and the uppermost surface of an epitaxial layer, similar to the case of a GaAs light-emitting device.
Provision of positive and negative electrodes in such a manner that they face each other via a light-emitting layer sandwiched therebetween is advantageous in that the area of a light-emitting region can be regulated to be approximately equal to the horizontal area of a support substrate, and that light extraction performance per unit device can be improved by virtue of attainment of light emission having uniform intensity. Prior art of the present invention is described in Japanese Patent Application Laid-Open (kokai) Nos. 2007-273492 and 2008-028291.
In the laser lift-off technique, when, for example, a structure including a sapphire substrate is irradiated with a laser beam having an appropriate wavelength, an aluminum nitride buffer provided on the substrate, and a gallium nitride layer formed on the buffer, GaN at the interface between the substrate and the gallium nitride layer is decomposed into molten metal gallium (Ga) and nitrogen (N2) gas; i.e., GaN at the interface between the GaN layer and the substrate is melted in the form of thin film. When, for example, a Group III nitride-based compound semiconductor is epitaxially grown through metal organic vapor phase epitaxy (MOVPE), the growth surface of the semiconductor is a so-called Ga-polar surface. Therefore, the surface of the aforementioned GaN layer exposed through the laser lift-off process is an N-polar surface. That is, when an n-electrode is formed on the GaN layer exposed through the laser lift-off process, the surface on which the n-electrode is formed is the N-polar surface of the GaN layer.
As has been well known, when an electrode is formed on a Group III nitride-based compound semiconductor layer (in particular, a gallium nitride (GaN) layer), ohmic contact is easily attained at a Ga-polar surface, but is difficult to attain at an N-polar surface. This phenomenon is theorized that nitrogen vacancies serve as pseudo-donors at the Ga-polar surface, and contact resistance between the Ga-polar surface and a metal which is in contact therewith is reduced, whereas virtually no nitrogen vacancies are present at the N-polar surface, and no reduction in contact resistance is expected at the N-polar surface.
When a Group III nitride-based compound semiconductor device is produced by bonding a conductive substrate (support substrate) to a p-electrode via a low-melting-point alloy layer (e.g., a solder layer) through the laser lift-off process, the following problems arise upon formation of an n-electrode.
First, since the low-melting-point alloy layer (e.g., solder layer) has been already used for bonding of the support substrate to the p-electrode, when, for example, formation of the n-electrode requires thermal treatment, heating to a temperature exceeding 400° C. cannot be carried out. Therefore, a material which requires annealing (i.e., thermal treatment) cannot be used for forming the n-electrode.
Specifically, thermal treatment at 350° C. to 400° C. after formation of the solder layer causes, for example, the following problem: when the solder layer is formed of a tin (Sn)-containing solder, tin (Sn) may be dispersed in a p-GaN layer or the support substrate made of silicon, and contact resistance may be increased between the p-GaN layer or support substrate and a metal layer formed thereon. Therefore, a material which requires thermal treatment at 500 to 550° C. cannot be used for forming an n-electrode of a Group III nitride-based compound semiconductor device produced through the laser lift-off process.
Second, in the case where a Group III nitride-based compound semiconductor device is produced by using, as an electrode material, a metal (e.g., tantalum (Ta)) which realizes low contact resistance between the electrode and an n-GaN layer without requiring thermal treatment, when the thus-produced semiconductor device is subjected to thermal treatment (e.g., at 250 to 300° C. for several minutes) by, for example, a user thereof, the contact resistance may be increased. Therefore, such an electrode material is not desired for forming an n-electrode of a Group III nitride-based compound semiconductor device produced through the laser lift-off process.